1. Field of the Invention
This invention relates generally to electronic signal conditioning; and more specifically, to a method as well as circuitry for compensating for nonlinear response characteristics of loads, in particular capacitive loads, driven by an amplifier.
2. Description of Related Art
Frequently, in the control of mechanical devices, high voltage amplifiers are used to drive piezoelectric actuators or other high-capacitance devices, which serve, in turn, to apply mechanical forces to the mechanical devices being controlled. Actuators can be formed from stacks of piezoceramic wafers which appear basically as pure capacitive loads to the high-voltage (driving) amplifier.
In certain instances the nonlinear response characteristics, exhibited to varying degrees by all ceramic composition capacitors, can be ignored, particularly in those applications where the capacitive load is being operated over a small range of voltage or where the nonlinear effects are unimportant. But in those applications where the capacitive load is being utilized for its piezoelectric properties of converting electrical energy to mechanical energy, the load must be driven over a large voltage range which exacerbates its nonlinear characteristics.
A few approaches to linearizing current through such purely capacitive loads exist. U.S. Pat. No. 5,000,415 discloses the use of charge and voltage feedback loops, positioned around the load so that the nonlinear voltage vs. displacement and the charge vs. displacement characteristics of the load tend to "cancel" each other. Also, in U.S. Pat. No. 4,628,275 a single feedback loop senses the charge from a large reference capacitor in series with the capacitive load. This last approach is not practical where the load capacitance is relatively large because the reference capacitor value becomes unreasonably large.
However, both approaches have in common the drawback that one or more control values must be fed back from the load to the controlling amplifier circuit. This can have at least two disadvantages. First, if the feedback value requires filtering to remove a switching component (such as in switching amplifier designs), signal phase shifts may be introduced. These must be compensated for by the control electronics, the net effect of which is a reduced control signal frequency bandwidth. Second, the physical design of the mechanical system which utilizes the capacitive load may not lend itself to having additional sensors and wires for the feedback loops from the load back to the controlling amplifier circuit.
Accordingly, there is a need to compensate for the nonlinear response characteristics of a capacitive load without the use of feedback loops from the load to the controlling amplifier, and without the need for relative large reference capacitors.